This invention relates generally to removable holding fixtures and particularly relates to a holding fixture of the type operated by a vacuum.
In Applicant's prior U.S. Pat. No. 4,056,334 a holding fixture of the vacuum operated type has been disclosed and described in FIGS. 6-9. It has been found that this holding fixture operates very well on solid flat surfaces such as a worktable.
However, in some cases a work bench or the like is covered with a formica sheet such, for example, as a low static or conductive polyethylene which will leak off electricity. A sheet of this type has the consistency of linoleum.
It has been found that the holding fixture of Applicant's prior patent above referred to, will not hold on such surfaces. The reason is that the fixture utilizes a single, large vacuum region and application of the vacuum to this region under the holding fixture will suck up the plastic sheet. Subsequently the sheet wrinkles and any wrinkles at the periphery of the vacuum region break the entire vacuum holding effect under the fixture. The situation becomes more aggravated in time because eventually the plastic sheet forms permanent wrinkles and the vacuum more readily pulls up the sheet, thus more rapidly destroying the vacuum.
It is accordingly an object of the present invention to provide a holding fixture characterized by a plurality of suction assemblies, each being independent of the other and each operating as an automatic valve.
Another object of the present invention is to provide a holding fixture which is simple in construction, reliable in operation and inexpensive to manufacture.